This invention relates to a receiver device of multilevel amplitude modulated signals and, more particularly, to a receiver device for demodulating transmitted signals guadrature amplitude modulated.
This type of receiver is provided with a coherent detector for converting transmitted signals into baseband signals and, a demodulator for deriving multi-level datum from baseband signals.
A conventional demodulation of the type described comprises an adaptive automatic equalizer which removes any intersymbol interference of baseband signals supplied from the coherent detector and feeds equalized signals to a discriminator predicting a transmitted datum. In the automatic equalizer, there is prepared a pair of delay lines with taps each of which is responsive to an in-phase and a quadrature phase components of the baseband signals, respectively. By controlling each tap gain of the delay lines, the amplitude and phase of the signal received can be simultaneously controlled therein.
However, when a carrier within transmitted signals undergoes any frequency offset and any other phase shift jitter, in a transmission line or a MODEM, the automatic equalizer cannot sufficiently absorb these fluctuations or phase variations.
Heretofore, to reduce the influence of phase variation, it has been proposed to feed back phase errors derived from correlation between outputs of the automatic equalizer and predictive values of transmission datum to a voltage controlled oscillator of the coherent detector. But, this proposal is too slow, in comparison to speed of the system because of the delay time from edge taps of the equalizer to central taps thereof.